1. Field of the Invention
The present invention relates generally to high performance polymeric materials. The present invention relates particularly to polyimide copolymers that are useful as adhesives, composite matrices, moldings, films and coatings.
2. Description of Related Art
Wholly aromatic polyimides are known for their exceptional thermal, thermooxidative and chemical resistance, but are generally difficult to process as structural adhesives or composite matrices. Several polyimides such as Kapton.RTM.(DuPont), PI-2080 (Dow Chemical, licensed to Lenzing), XU-218 (Ciba-Geigy), Ultem.RTM.(General Electric) and LaRC.TM.-TPI (Mitsui Toatsu) are commercially available and used as fibers, films, moldings, adhesives or composite matrices.
Currently available equipment to process polyimides into useful parts are limited in their pressure and temperature capability, and certain applications require co-curing of adhesives and composites with other structures such as foams or honeycombs. Because of the equipment limitations (especially autoclaves), it is extremely important to provide materials that are processable at 200 psi or below and 371.degree. C. or below. Because of the foams and honeycombs that are being proposed for use in some applications, reductions in pressure below 200 psi are also very significant.
While improved processing conditions are very important, the polyimides must also display better mechanical and adhesive properties to meet the needs of many future applications. Especially important for these applications are properties measured at temperatures of 177.degree. C. or slightly higher, for use over long time periods at those elevated temperatures.
Thermoplastic polymers currently available are either difficult to process into high quality parts or have limited mechanical performance at the elevated temperatures for short or long periods of time. The related art that comes closest to meeting the needs of future applications is a thermoplastic polyimide known as LARC.TM.-IA, as described by St. Clair and Progar in U.S. Pat. No. 5,147,966. However, this polyimide requires higher processing conditions than desired and/or provides lower mechanical and adhesive properties than desired, depending on the tests performed.
A primary object of this invention is to provide polyimide copolymers which can be processed at low pressures to provide polyimides with improved solvent resistance, modulus and elevated use temperatures.
Another object is to provide polyimide copolymers with improved melt processability, that can be processed without the evolution of volatiles, that are melt stable at high temperatures, that have improved adhesive properties, and that have improved composite properties.